An aircraft (heavier than air vehicle) can be perceived as a two stage energy conversion vehicle. The first stage converts thermal energy to potential energy (altitude). The second stage is the conversion of potential energy to kinetic energy (motion). The power source converts the thermal energy to potential energy. The airframe is responsible for the efficiency of the conversion from potential energy to kinetic energy. Sailplanes, for example, represent the epitome of the potential to kinetic energy conversion process. That efficiency is measured by the glide angle of the airframe. That is, for a given altitude how far will the airframe go before landing? It can be understood that the airframe that generates lift with the lowest amount of fluid dynamic drag will go the farthest. With careful engine addition, the airframe retains most of its efficiency.
Fluid dynamic drag consists of three parts; parasitic drag (skin friction, profile, and various interference components), induced drag, and trim drag. Two of these drags have received considerable attention. Historically, a great deal of attention has been paid to parasitic and induced drag, but relatively little attention has been paid to reducing trim drag. Herein, trim drag is considered to be the energy consumed by an airframe in flight as needed to sustain balance in level flight or attitude in accelerated maneuvers (i.e. turns, pullouts, aerobatic maneuver, etc.). It is the intent of this invention to provide unique means of trim drag reduction.
This invention relates to airframes and particularly to airframes of the single wing, flying wing, and/or tailless wing type, or in other words, to the class of airplanes in which the functions of the usual tail structure or empennage are accomplished by structures attached to or incorporated within the outlines of the main supporting airfoils. It is to be recognized that whereas this invention relates to airframes, said invention is applicable to both powered and unpowered aircraft. For the purpose of discussion throughout this patent application including claims, the term "tailless" is used to include flying wing or tailless wing airframe configurations.
Airframes of the tailless wing type have heretofore been proposed and constructed with the idea that airframes of this type avoid undue aerodynamic drag so that the efficiency of the airframes would be improved. In such airframes of the tailless type, it has been found that a high degree of longitudinal stability might be attained, but such stability in a longitudinal sense has, however, been attained in the prior airplanes of this type only through inefficient means which have reduced the overall efficiency of such airplanes so as to render them undesirable in a commercial sense. Energy consumed for lateral and rolling control have similarly been excessive relative to conventional airframe configurations with an empennage.
Specifically, the aforesaid longitudinal stability has been obtained by means that have generated excessive trim drag. The excessive trim drag in tailless type airframes has been the result of either two individual primary causes or a combination of these two primary causes. One, reflex cambered (stable) airfoils that can provide longitudinal stability through aerodynamic generated nose up pitching moments but at the expense of limited maximum lift coefficients relative to those lift coefficients available through unreflexed (unstable) airfoils have been used. The limited maximum lift coefficient can be recognized as a trim drag penalty necessary for longitudinal stability. This cause of excessive trim drag is most common in unswept tailless type airframes.
The other primary cause of trim drag is found in aft swept tailless aircraft. It is the result of a downward acting balancing force (negative span loading) near the outboard wing tips aft of the center of gravity for the purpose of providing longitudinal stability. In order for the aft swept type of flying wing to maintain level flight, the primary lift of the wing must equal the weight of the airframe plus the magnitude of the downward acting, balancing force. Therefore, more lift is generated than is actually necessary to sustain flight. The extra lift generates extra drag, i.e. trim drag. NASA reports NASA-TN-D-8260 and NASA-TN-D-8264 demonstrate that the use of winglets reduce induced drag when properly applied to a wing by increasing the span loading in the outboard portions of the wing and by lift forces generated directly upon the individual winglet by the wing tip vortex.
U.S. Pat. No. 4,245,804 issued to Mr. Ishimitsu on Jan. 20, 1981; U.S. Pat No. 4,240,597 issued to Mr. Ellis, Mr Gertsen, and Mr. Conley on Dec. 23, 1980; U.S. Pat. No. 4,205,810 issued to Mr. Ishimitsu on June 3, 1980; U.S. Pat. No. 4,190,219 issued to Mr. Hackett on Feb. 26, 1980; and U.S. Pat. No. 2,576,981 attest to the feasibility and desireability of using a fixed airfoil at the end of a wing for the combined purpose of generating a force and controlling wing span loading in a manner so as to reduce airframe induced drag.
In a tailless airframe, further benefits may be realized. U.S. Pat. No. 2,474,585 teaches the use of a rotatable vertical wing-tip panel for generating drag sufficient to overcome engine out yaw. It is the intent of the herein disclosed invention to teach that non-planar, airfoil shaped, wing extensions (winglets) may be used to stabilize and control a tailless aircraft by utilizing the lift force the winglets individually generate and by utilizing the winglet control over a portion of the outboard wing span lift distribution.
Pitch stability and control, as well as yaw and rolling control may be obtained by the use of controllable winglets properly arranged so as to vary a portion of the wing span lift distribution and the side (lift) force generated by the airfoil shaped winglets which are mounted so that in their normal positions they are able to effect minimum induced drag. Trim drag is significantly reduced relative to conventional aircraft (with tails) and canard configurations because the majority of stability and control functions are assigned to the controllable winglets therein allowing an airframe to be commercially efficient without the trim drag penalties associated with conventional tails and canards.
Winglets, properly installed for induced drag reduction, generate an inboard acting lift force (for a non-planar winglet rising above the wing plane) and generate an increase in wing span lift distribution (whether the winglet descends, is in plane with, or rises above the plane of the wing). In a forward swept tailless airframe, positive, (upward) span lift in the outboard portion of each wing panel is necessary for airframe balance. The use of winglets provide a very useful, favorable, and controllable portion of wing span lift distribution working in harmony with the winglet generated lift force. But in an aft swept wing tailless airframe, balance is obtained with a negative (downward) wing span lift distribution. Therefore, the winglet influence wing span lift distribution is contradictory to the desired use of the winglet for simultaneous minimum induced drag. Therefore, while winglets for control of aft swept tailless aircraft may be used, the preferred minimum induced drag configuration is the forward swept tailless aircraft.
Though there are diminishing returns, one or more winglet wing extensions at varying cant angles (including in wing plane) may be used to influence the wing span load distribution. The preferred arrangement consists of two winglets extending from each wing tip. Accordingly, it is an object of this invention to provide a substantially non-planar tailless type airframe configuration which exhibits improved aerodynamic efficiency with respect to trim drag. Higher lift coefficient producing airfoils (Typically unstable, i.e. nose down pitching moment) and high lift devices such as trailing edge flaps maybe used. Suitable choices of the variables within the concept of the invention allow for flight at supersonic as well as subsonic velocities.